She lurked in the lake of Lerna, aware but slumbering. Souls drifted past her down into the Underworld. It bored her, they were always going by, and were no fun to torture, since they were already dead and unable to feel pain. Not many living folk came by here as they were all terrified of her. So when anyone was to wander by, she'd take her time. She was so poisonous that she killed men with her breath, and if anyone passed by when she was sleeping, he breathed her tracks and died in the greatest torment.
Her many heads lifted a little at the sound of feet in the grass. She smiled viscously and sank beneath the water without a ripple or a sound. The man was whistling cheerfully, and called out to her, asking for her to grant him a favour. This confused her; she had never done anything to help anyone but herself, where did this mortal get the idea that she would help him? Her heads burst out of the lake, and she hissed menacingly. The man trembled but held his ground, she was careful not to breathe on him, lest he die without telling her his reasons. He looked up and seemed unsure of which pair of eyes to look into, which was amusing as his eyes flickered from one head to another.
"Great beast, which is your main head?" He asked, and she grinned. She moved all of them, and his confusion deepened. "Am I to understand that each head is a dominant head?" She nodded them all and he smiled a tiny bit. She hissed at him, and he took half a step back. "I wish to ask a favour of you." He said, and she narrowed her many eyes. "There is a man who will walk by here today, on the pretence that I will meet him here to return some money to him. Will you kill him for me great beast?" She thought a moment and a plan formed. She nodded her heads and he told her he would arrive an hour before sunset. She slipped beneath the waters and laughed to herself, anticipating an enjoyable evening.
When the man turned up, he asked her to remain hidden until his signal. She hid, and his target arrived a few minutes after. They talked and the new man threatened the first. She burst up and poisoned the new man before he could even scream.
"Thank you great beast!" The man cried, and began to walk away. She smiled sadistically and one of her heads shot forward and grabbed his leg, dragging him back to the lake. He screamed and pleaded, and she entertained herself with great satisfaction.
When both men were being digested, and she was comfortable and content, the Hydra settled down for the night, both aware and slumbering, while two new souls were added to the mass of the dead going down to the Underworld.